According to the U.S. Census Bureau, the county has a total area of 976 square miles (2,530 km2), of which 574 square miles (1,490 km2) is land and 402 square miles (1,040 km2) (41%) is water.[5] It is the second-largest county in Mississippi by total area.

There were 71,538 households out of which 33.50% had children under the age of 18 living with them, 48.10% were married couples living together, 15.10% had a female householder with no husband present, and 32.10% were non-families. 25.80% of all households were made up of individuals and 8.30% had someone living alone who was 65 years of age or older. The average household size is 2.55 and the average family size is 3.07.

In the county the population was spread out with 26.00% under the age of 18, 11.10% from 18 to 24, 30.50% from 25 to 44, 21.20% from 45 to 64, and 11.10% who are 65 years of age or older. The median age was 34 years. For every 100 females there were 99.10 males. For every 100 females age 18 and over, there were 97.50 males.

The main income for a household in the county was $35,624, and the median income for a family was $41,445. Males had a median income of $29,867 versus $22,030 for females. The per capita income for the county was $18,024. 14.60% of the population and 11.60% of families were below the poverty line. Out of the total population, 20.70% of those under the age of 18 and 11.30% of those 65 and older were living below the poverty line.

Harrison County has the sixth highest per capita income in the State of Mississippi.

Harrison County has been studied by CNN and other media, which have reported on the beatings of inmates in the Harrison County Jail in Gulfport, Mississippi. Inmate Jessie Lee Williams Jr. died while in custody on February 4, 2006.[13] In 2006 and 2007, six Harrison County Sheriff's Department deputies pleaded guilty to crimes related to the abuse of inmates at the jail.[14] Sheriff Melvin Brisolara-R was elected in 2008, for Harrison County.